1. Field of the Invention
The present invention relates to an improved method to degrade cellulose-containing fluids of the type used during completion, workover and fracturing operations at moderate to high temperatures or at an alkaline pH range.
2. Description of the Prior Art
Cellulose-containing fluids are used downhole in a number of wellbore operations, such as workover, gravel packing, and cementing. During hydraulic fracturing, another type of wellbore operation, a sand laden fluid is injected into a wellbore under high pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. This viscosity is normally obtained by the gelation of suitable polymers, such as a suitable polysaccharide, and are known as fracturing gels. The gelled fluid can be accompanied by a propping agent which results in the placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the wellbore into the formation being treated once the fracturing fluid is recovered. Propping agents include a wide variety of material and may be coated with resins. The gel fluids may also contain other conventional additives common to the well service industry such as surfactants, and the like.
Occasionally, production from wellbore operations must cease temporarily to perform auxiliary procedures called workover operations. The use of temporary blocking gels, also formed by gelation of appropriate polysaccharides, produces a relatively impermeable barrier across the production formation.
Polysaccharides have other uses within the oil industry. For example, uncrosslinked cellulose-containing polysaccharides thicken fluids and control fluid loss. They are used with proppants, such as sand control fluids and completion fluids, such as those for gravel packing.
Gravel packing controls sand migration from unconsolidated or poorly consolidated formations through the placement of a gravel pack around a slotted or perforated liner or screen liner inserted at a specific location within a perforated wellbore. The "gravel" is usually sand or a very fine gravel that excludes the formation sand from entering the wellbore. Celluloses are typically used to thicken the fluids in order to properly pack gravel into the perforations of the wellbore. Although unthickened slurries pack an annulus well, the sand compacts quickly and may not have sufficient time to flow into and completely pack the perforations.
The viscosity of all of these types of cellulose-containing fluids, whether crosslinked or not, must be reduced at the end of the operation. At the end of fracturing or workover operations for example, the gels are degraded and the fluids are recovered. Gel fluids are recovered by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion of gels is referred to as "breaking" and is often accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
A similar reduction of the fluid viscosity of uncrosslinked, cellulose-containing fluids occurs at the end of completion operations. For example, at the end of gravel packing, the viscosity is reduced to allow the settlement of the sand to properly pack the annulus. Therefore in this disclosure, "breaking" refers to the reduction of viscosity of a cellulose-containing fluid, whether crosslinked or uncrosslinked, to a low value such that it flows from the formation under the influence of formation fluids and pressure.
In addition to the importance of providing a breaking mechanism for the fluid which facilitates recovery of the fluid and resumes production, the timing of the break is of great importance. Gels that break prematurely can damage the production zone through the leak-off of contaminating materials into the production formation. If the viscosity is reduced prematurely during gravel packing, the sand settles before being properly placed within the wellbore and perforations, thus contributing to the problem of sand within the wellbore.
On the other hand, fluids that break too slowly can cause slow recovery of the fluid from the production formation. Slow recovery delays the resumption of the production of formation fluids and can cause improper packing the annulus during gravel packing. Incomplete gel degradation causes a build up of residue which interferes with production from the formation.
For purposes of the present application, premature breaking means that the viscosity diminishes to an undesirable extent prior to the end of the operation. Thus, to be satisfactory, the fluid viscosity should remain in the range from about 60% to 100% for the length of time required to complete the operation. Since some operations require extended periods of time before completion, the fluids should be capable of remaining appropriately viscous during that time period. In the laboratory setting, viscosity is measured using a rotational viscometer such as a Fann 35VG meter or a Brookfield DVII digital viscometer.
For practical purposes, the viscosity of the cellulose-containing fluid should be completely reduced within a specific period of time after completion of the operation. This period of time depends on the temperature of the formation. Optimally, a gelled fluid breaks when the operation concludes. A completely reduced fluid means one that can be flushed from the formation by the flowing formation fluids. A completely broken, uncrosslinked gel regains greater than about 95% of the initial permeability of a formation sample using a gel damage permeability test.
Enzyme systems are known to degrade the types of polysaccharides used in fracturing and blocking gels as well as other oil industry applications. Enzyme breaker systems have been designed to break gelled fracturing and blocking fluids used in the industry as well as filter cakes. See, for example, U.S. Pat. No. 5,224,544 "Enzyme Complex Used for Breaking Crosslinked Cellulose Based Blocking Gels at Low to Moderate Temperatures" and U.S. Pat. No. 5,247,995, "Method of Dissolving Organic Filter Cake Obtained from Polysaccharide Based Fluids Used in Production Operations and Completions of Oil and Gas Wells", assigned to the assignee of this invention and incorporated herein by reference. The cellulose enzyme treatment disclosed in these patents effectively degrades cellulose-containing fluids at moderate conditions. The cellulose enzymes degrade specific cellulose linkages at pH ranges from about 1.0 to about 8.0, with an optimum pH ranging from about 3.0 to 4.0 at low to moderate temperatures of about 10.degree. C. (50.degree. F.) to about 60.degree. C. (140.degree. F.). However, the disclosed enzyme treatment is less effective at pHs above about 8, as well as temperatures above about 60.degree. C. (140.degree. F.). Like most enzymes, the efficacy of the above-disclosed enzyme system is reduced at pHs at the high end of the activity range, in this case between about 7 and 8, while the stability and activity of the enzymes decreases at the higher temperatures.
Therefore, the present invention has as its object to provide a mechanism for degrading cellulose-containing fluids used during fracturing, workover and completion operations to produce an efficacious degradation of the cellulose-containing fluid at an alkaline pH range. In addition, the present invention has as its object the ability to effectively degrade cellulose-containing fluids at moderate to high temperatures.
Another object of the invention is to provide an enzyme system that degrades the cellulose-containing fluids into primarily monosaccharide and disaccharide fragments at alkaline pH ranges and moderate to high temperatures.
Another object of the invention is to provide a mechanism for degrading cellulose-containing fluids at alkaline pH ranges and moderate to high temperatures that does not react with other materials or metals used in wellbore operations or found within the subterranean formation.